It is well known that flat metal strip can be formed to a predetermined profile across its width that is uniform along its length by passing it lengthwise through a succession of forming roller pairs by which it is progressively worked to the desired transverse profile.
The advantages of roller forming are also well known. A high rate of production is obtained because the metal moves continuously and at relatively high speed as it is converted from flat strip to finished workpieces. Since the process is essentially continuous and automatic, labor costs are minimal; and the equipment needed for roller forming is relatively inexpensive in relation to the high rate of production that it can achieve.
The limitations upon roller forming are also well known. One of these is that only a limited depth of transverse bend or curvature can be imparted to a workpiece by roller forming. For example, an attempt to bring a workpiece to a substantially deep V-shaped or U-shaped transverse profile by roller forming would result in unacceptable scuffing of the workpiece because the radially outer portions of the final-stage forming rollers would have surface speeds much higher than their radially inner portions.
Roller forming can also present problems with respect to cutoff of individual finished workpieces from a roller formed strip. For some operations in which lengthwise moving strip is cut into workpieces it is possible to use a press having a so-called flying die set, that is, a die set which is propelled for motion with the advancing strip while shearing through it and which is thereafter moved oppositely to the direction of strip motion, back to a home position. With such a press there is no need to stop forward motion of the strip each time a cutoff operation is performed, and therefore the strip can maintain the steady, continuous forward movement that is imparted to it in the roller forming process. However, where a flying die set must cut through strip that has been roller formed, the cooperating shearing dies must be curved to match the transverse profile imparted to the strip by the forming rollers, so that the dies will not deform the end portions of each workpiece out of that profile. Substantial costs are involved in grinding such profiled shearing dies and regrinding them when they need sharpening, and therefore it has heretofore been considered feasible only to shear through roller formed strip with dies that produce straight end edges on the workpieces. Grinding the dies to have their edges curved in two dimensions--to match a transverse profile and to produce a curving end edge--would entail unacceptably high costs.
On the other hand, cutting flat strip into individual workpiece blanks and then passing the blanks through forming rollers is seldom practicable. A piece of metal to be roller formed must have adequate length, in order to be controlled against edgewise sideward movement as it passes from one to another of the forming roller pairs. Ordinarily, control cannot be maintained unless the piece is long enough to be in simultaneous engagement with at least three pairs of forming rollers.
A typical workpiece that presents all of the above mentioned problems is a component of a reflector-diffuser assembly for a fluorescent lighting fixture. The assembly, which fits into the fixture in front of the fluorescent tubes to diffuse their light, comprises a rectangular frame that supports a grid-like array of reflecting elements or louvers. Each louver is folded along its longitudinal centerline to have a narrow V-shaped transverse profile, but each leg of the V is curved across its width to provide for effective light reflection and diffusion. The array comprises a set of longitudinal louvers that are spaced apart at regular intervals across the width of the assembly and a set of transverse louvers that are spaced apart at similar intervals along its length. The louvers of each set are slotted to receive those of the other set, so that the two sets of louvers interengage like the partition members in an egg crate. The frame that supports the louvers comprises end walls and side walls, each curved across its width, and therefore each louver must have its opposite end edges curved to match the transverse profile of the frame walls between which it extends. In addition, there must be small tabs or lugs on each louver, projecting endwise from its end edges, to be received in slots in the respective frame walls, for holding the parts assembled.
Although the louvers of a fluorescent light fixture diffuser-reflector can be made from thin-gage strip aluminum that lends itself well to roller forming, it has not heretofore been considered feasible to employ roller forming in the production of such louvers. They were therefore produced by a time-consuming and labor-intensive procedure wherein individual blanks were first punched out of sheet or strip stock, and then each blank was subjected to a succession of press forming operations to bring it to the desired profile. For production continuity, a separate press was used for each forming operation, and blanks were transported from press to press in batches. To preserve the shiny, reflective surface of the metal, the blanks in each batch were separated by sheets of paper that prevented them from scratching and scuffing one another. In addition to the hand operations at each press that were required for stacking the blanks and their paper separators, the blanks had to be individually loaded into each press and removed from it. It is evident that those skilled in the art could find no obvious method or means for reducing the high labor costs and the large capital investment in press equipment that were needed with this prior procedure, since the resultant high price of the product was an open invitation to devastating competition from areas where labor is cheap.